The present invention relates to hydraulic control systems, and more particularly, to those control systems of the “load sensing” type, i.e., a hydraulic control system in which a load signal is generated which is representative of the hydraulic load on the circuit, with that load signal being utilized to control, or at least vary, the rate of fluid delivery of the source of pressurized fluid for the circuit.
Although the present invention may be utilized with hydraulic control circuits for a number of different applications, whether on a mobile vehicle, or on a stationary (or industrial) hydraulic system, the invention is especially advantageous when used in conjunction with a hydrostatic power steering system for a mobile vehicle, and will be described in connection therewith. Furthermore, the present invention is of special benefit when used as part of a hydrostatic power steering system to steer a vehicle, such as an articulated vehicle, in which the load on the steering actuator is quite substantial, or the steering “inertia” of the vehicle is very large.
A hydrostatic power steering system of the type which would utilize the present invention would typically include a pump (fixed or variable), a load sensing priority flow control valve (“LSPV”) which apportions flow between a priority load circuit (in this case, the steering circuit) and an auxiliary load circuit (another vehicle hydraulic function) in response to variations in a load signal representative of the hydraulic load on the priority load circuit. Flow from the LSPV to the steering actuator (typically, one or more steering cylinders) is controlled by a steering valve such as the Orbitrol® steering control unit (SCU) sold commercially by the assignee of the present invention.
As is now well known to those skilled in the art, a conventional SCU, of the type used in a load sensing circuit, defines various flow control orifices which are closed when the SCU is in its neutral condition (no steering input) and the various flow control orifices begin to open as the operator rotates the steering wheel in either direction from the neutral condition, to select either a right turn or a left turn.
It has been observed that on vehicles such as large, articulated vehicles, if the vehicle operator suddenly stops the steering input to the SCU, or suddenly reverses steering direction, one result which is likely to occur is a substantial pressure spike in the conduits interconnecting the SCU and the steering actuator, which is likely to impose a severe “jerking” movement to the vehicle. These pressure spikes, which need to be relieved, are caused in part by the momentum of the steering actuator, and the fluid in the lines between the actuator and the SCU being trapped, because of the SCU control orifices closing. The resulting jerking movement can be extremely undesirable from the viewpoint of operator safety and comfort.
For years, those skilled in the hydraulics art have utilized valves of the type commonly referred to as “cushion valves” to deal with the problem of pressure spikes in hydraulic lines, especially in lines between a control valve and an actuator. Examples of such cushion valve arrangements are shown in U.S. Pat. Nos. 3,330,298 and 4,040,439, both of which are assigned to the assignee of the present invention and incorporated herein by reference. Although such cushion valve arrangements have been used successfully in various applications, such valves typically operate on the “pressure rise rate” principle, as is well known to those skilled in the cushion valve art, thus limiting their usefulness in hydraulic systems (and particularly in steering systems) subjected to the “sudden stop” or “sudden reversal” types of condition described above. Another deficiency of cushion valves is that during every steering motion flow from the SCU work ports is used to shift a valve spool of significant size, thus resulting in some lost motion. Such lost motion can lead to control difficulties at higher vehicle speeds. Furthermore, the typical cushion valve arrangement known in the prior art has been very complex and expensive, thus further limiting the commercial potential of such valves.